Optical inspection systems for detecting particles on the surface of a semiconductor substrate and for measuring surface roughness of a semiconductor wafer are well known. Typically, a detection beam is scanned across the surface of a wafer and a collector is used to collect light from the semiconductor substrate. The detector beam is directed at a 90.degree. angle relative to the semiconductor surface under test. A smooth surface provides a predictable intensity of reflected light from the surface. However, surface roughness and foreign matter cause a scattering of the radiation and a change in the light that is received at the collector.
Optical inspection systems for semiconductor wafers must be tested and should be periodically calibrated to ensure uniformity of system sensitivity over time. A device and a method for calibrating a system used in determining surface roughness and detecting foreign matter on a semiconductor wafer are described in U.S. Pat. No. 4,512,659 to Galbraith et al. A beam is projected onto a surface having arrays of artificial defects. The artificial defects may take the form of a pit in the surface or a raised mesa on the surface. The artificial defects scatter the incident light, thereby simulating pits and raised areas on a semiconductor wafer. The spacing between artificial defects is less than the diameter of the incident beam. The surface having the array of artificial defects can be used as a reference device whenever an inspection system requires recalibration.
In addition to the use of optical inspection systems for determining surface roughness and foreign matter on a semiconductor wafer, such systems are used in determining the presence and location of particles on a masking device employed in photolithographically fabricating integrated circuits on the semiconductor wafer. Masking devices are herein defined to include reticles and pellicles. A pellicle is a thin transparent film that is spaced apart from a reticle or the like. Any particles that would otherwise come to rest on the surface of a reticle are instead seated on the pellicle. The particles can then be maintained outside of the focal plane of the apparatus used in transferring the pattern on a reticle to a semiconductor wafer.
There are a number of differences between optical inspection systems for semiconductor wafers and systems for inspecting a reticle or pellicle. In wafer inspection, the angle of incidence of an incident beam is often approximately 90.degree. to the wafer. Pellicle inspection is typically associated with a grazing incidence angle, i.e., one in which the detection beam is nearly parallel to the pellicle surface. Thus, particles on a pellicle can cause "vignetting," or shadowing, of other particles. Vignetting can be a result of a first particle being directly behind a second particle with respect to the angle of incidence of the detection beam or with respect to light that would otherwise reach a collector after being scattered by the second particle.
An object of the present invention is to provide a test device for calibrating an optical surface inspection system in which the angle of incidence of the detection beam is less than 30.degree., wherein the test device is less susceptible to vignetting and is easily manufactured, handled and cleaned.